Chromium and chromium master alloys are widely used in production of "super" alloys. Some manufacturers of such super alloys desire extremely low limits of nitrogen in the master alloys in order that the ultimate alloy not have unacceptable impurities and inclusions resulting from nitrogen. For example, one super alloy manufacturer specifies that chromium master alloys have a maximum of 0.003%, by weight, nitrogen. Such products were unknown in the commercial marketplace prior to the present invention with typical commercially available chromium containing as low as only 0.008 to about 0.03% nitrogen. It was recognized in the art that this rather high nitrogen content occurred as the result of reaction of chromium with the atmosphere (air), during production.
There may have been attempts to produce low nitrogen chromium and chromium master alloys by refining under vacuum conditions to reduce the nitrogen content of the chromium or chromium alloys; however, such attempts are not known to have been successful or practical means of reducing the nitrogen content of the chromium or chromium master alloys to acceptable levels.
A water cooled copper vessel has found wide use for production of high purity master alloys by the thermite process. A form of water cooled copper vessel is described by applicant in Trans. Met. Soc. AIME 1967, Vol. 239, pp. 1282-1286. It is practical to remove nitrogen from a thermite system to be reduced in such a vessel since the vessel can be made vacuum tight. If this process were followed in the aluminothermic reduction of chromium oxides, it should preclude high nitrogen content in the resultant product by eliminating pick up of nitrogen from the atmosphere during the reduction, cooling and solidification periods.
However, the water cooled copper walls of such vessels generally limit their usefulness to reduction of those metals or alloys melting at about 1650.degree. C. or lower. Chromium metal melts at about 1880.degree. C., and was found to splash about the copper vessel during reduction. This caused a thin layer of chromium to coat the walls of the vessel above the reduction zone. This thin layer of metal is commonly referred to as an armour plate.
It is the object of this invention to produce chromium-nickel master alloys having a nitrogen content of less than 0.005%, preferably as low as 0.001% to 0.003%, by weight. It is a further object of this invention to produce such master alloys in water cooled copper vessels after vacuum degassing the mix and subsequent reaction (reduction) under an inert gas atmosphere.